1. Field of the Invention
The invention relates to a process for coating fibers by dipping them into liquid metal and, more particularly but not exclusively, to a process making it possible to obtain a thick and uniform coating of metal that can have a high melting point and a high reactivity, such as titanium alloys.
2. Discussion of the Background
The most promising materials for reducing the mass of products intended for the aeronautics or space industry, particularly turbojets, are metal-matrix composites consisting of a matrix based on a metal alloy reinforced by fibers, said fibers being, for example, ceramic fibers. These materials have a high level of performance in terms of stiffness and strength, and they can be used instead of monolithic alloys for producing components such as compressor or turbine disks, shafts or actuator bodies, it being possible for these materials also be to be used as local reinforcements in blades, casings, spacers, etc. The mass savings achieved with the components vary from 15 to 70%, depending on the applications, their overall size is reduced and their lifetime significantly increased. These materials are also useful in other sectors in which a field of volume forces is applied to a component, for example a pressure shell such as a gun barrel or a tank for fluid under pressure.
Patent FR 2,684,578 describes a process for manufacturing components made of a metal-matrix composite material which uses ceramic fibers coated with the material of the metal matrix, these fibers then being wound and pressed at the isothermal forging temperature of the matrix material. The coating may be carried out in the vapor phase in an electric field, by electrophoresis using metal powders, or by dipping them into a bath of liquid metal. The ratio between the fiber volume and the total volume of the fiber+matrix material is typically, but not necessarily, approximately 30%, which leads to the fibers being coated with a layer of metal whose thickness is very much greater than that of an ordinary surface treatment. For example, a 100 .mu.m diameter fiber must, under these conditions, be coated with a thickness E of metal equal to 35 .mu.m. Moreover, the thickness of the coating must be very uniform so as to guarantee a constant separation of the fibers in the composite material. If this were not the case, stress concentrations would be produced under load, which would substantially reduce the fatigue strength of the composite material, with the fibers breaking or the matrix cracking. This well-known phenomenon stems from the large difference between the Young's moduli of the fiber and the matrix material. Typically, silicon carbide SiC ceramic fibers have a Young's modulus of about 500 GPa whereas a matrix made of a titanium alloy such as TA6V has a Young's modulus of about 100 GPa. Finally, such materials with a high strength/mass ratio are of great interest, especially for the aeronautics and space industry, but their high manufacturing cost constitutes a brake on their development, this high manufacturing cost stemming both from the cost of the materials used and from the complexity of the known manufacturing processes employed.
In order to coat the fibers, the use of the vapor deposition process in an electric field has several drawbacks which make it not very profitable from an industrial standpoint:
This process is very slow since the rate of metal deposited is low while the coating to be produced is thick. PA1 A large amount of metal is deposited on the instruments and the walls of the chamber. Not only is this metal lost, but the work must also often be interrupted in order to clean the plant. PA1 The plant is expensive and consequently can be amortized only over major production runs. PA1 Finally, only certain metal alloys can be used with this coating method. PA1 a. a feed opening is placed in the bottom of the crucible and below the inductor, said feed opening facing downward; PA1 b. a bar of metal is introduced via the feed opening and the upper end of said bar is raised into the crucible inside the inductor; PA1 c. the liquid metal is shifted away from the edge of the feed opening by the levitation effect, said shift taking place at least over a distance equal to the gap E1 left between the bar and the edge of the feed opening.
The dip-coating process is the most rapid in its principle and U.S. Pat. No. 5,049,419 gives an example of its implementation: The fiber passes through the liquid metal held in a crucible, this fiber being kept tensioned by three idler pulleys, one of which is immersed in the liquid metal. However, this process has the drawback of being difficult to use with alloys having a high melting point and/or reactivity, since the ability of the idler pulley and, more generally, of the mechanical components immersed in the liquid metal to withstand such conditions is very limited or even nonexistent. It is known to protect the internal wall of the crucible by an oxide such as alumina, but only yttria Y.sub.2 O.sub.3, which is very expensive, can be used in the presence of a titanium alloy. Moreover, an oxide coating does not solve the problem caused by the pulley being immersed in the liquid metal.
The so-called "Landau's" law, which allows the thickness of the metal layer thus deposited on the fiber to be determined as a function of various parameters, is known. However, this law does not properly apply to all the ranges of values that these parameters may have in an industrial process. This law is applicable for low unwinding speeds of the wire.
Also known, for example from Patent FR 2,649,625, is a so-called "cold" crucible in which the liquid metal is held in levitation by an electromagnetic field produced by a helical electromagnetic inductor surrounding the crucible. Such crucibles are designed and used to prevent contamination of the liquid metal by the material of which the wall of the crucible is made. Also known, from a summary of Japanese Patent JP 4099160, is a process for coating a tape with metal, the said process consisting in making said tape run vertically into an induction crucible containing liquid metal, said tape passing through an orifice located at the bottom of the crucible and through the opening above the crucible, said orifice being shifted toward the edge of the crucible and said liquid metal being moved away from the orifice by a partial levitation created by the inductor. An advantage of this process is that it makes it possible to dispense with any mechanical member immersed in the liquid metal. A first drawback of this process is that it makes it difficult to introduce the tape into the liquid metal. A second drawback of the process is that it gives the metal layer deposited on the surface of the tape a nonuniform thickness, since the tape runs through the surface of the liquid metal very obliquely.